This application is concerned with the field of nondestructive testing and, more particularly, with ultrasonic methods for measuring the thickness of materials.
Nondestructive testing is a branch of materials science which is involved with all aspects of the quality and uniformity of materials. Among the various analytical tools which are available in the discipline of non-destructive tests, ultrasonic techniques have proven useful for a variety of measurement tasks. Ultrasonics may be applied, for example, in the research and development stage to identify material variables. Ultrasonic techniques are also available for use in quality control procedures during the production of an item, and as process control measures to ensure the uniformity of a continuously produced product. Ultrasonic devices and methods may further be employed for the on-site inspection of installed systems and for the examination of in-service components to detect such failure parameters as wear, deterioriation, and corrosion.
A particular area of materials testing in which ultrasonics has proven valuable is in performing material thickness measurements. The thickness of an object may be measured with very high accuracy by ultrasonic techniques. In addition, the use of ultrasonic thickness measurement techniques makes possible the measurement of otherwise unmeasurable components, such as, for example, where physical access to the component is limited so that a direct mechanical measurement cannot be accomplished.
A resonance method for measuring thickness using ultrasonic waves is known in the art. See, e.g., Nondestructive Testing--Views, Reviews, Previews (H. Egerton ed. 1969) at pp. 46-56. In the resonance technique, an ultrasonic wave is generated in the object whose thickness is to be measured, and the frequency of the generated wave is varied until maximum resonance is observed. The thickness of the object is then calculated by correlating the thickness to a multiple of the wavelength of that ultrasonic wave exhibiting maximum resonance. The resonance technique, however, is subject to some shortcomings and disadvantages. The resonant frequency for a particular material or a certain thickness, for example, may be too high to be generated by available ultrasonic transducers. This limitation is particularly applicable in the case of relatively thin plates and tubes. In addition, some transducers, such as the electromagnetic acoustic transducer designs, do not exhibit a sufficiently short response time to operate effectively with the resonance method. Furthermore, the resonance technique will provide a thickness measurement which is limited to a localized area of an object. If a generalized measurement over a broader area of the object, such as a length of tubing, for example, is needed, an area of the object to be tested must be covered with a sequence of resonance measurements at relatively closely spaced surface intervals in order to effectively cover the broader area.
Consequently, a need has developed in the art for an ultrasonic method which is capable of performing an area measurement of thickness.
In addition, a need has developed for an ultrasonic thickness measuring technique capable of measuring the thickness of relatively thin plates and tubes while incorporating the use of electromagnetic acoustic transducers.
Moreover, there is a need in the art for an ultrasonic thickness measuring method which may be utilized to measure an average thickness or to measure a minimum thickness in a preselected area of an object.